Roots pumps are applied in semi-conductor manufacturing processes and liquid crystal panel manufacturing equipment includes rotors mounted on a pair of revolving shafts, respectively, to transport and discharge gas from pump chambers with sequentially decreasing volume.
In order to reduce power consumption when this kind of multistage roots pump operates at maximum operating pressure, it is necessary to reduce the discharge volume at back stage (downstream side of the gas travel path) especially at the final stage. The discharge volume is determined by the volume of space formed by valleys of rotors that have multiple teeth, and the internal surface of pump chambers where rotors are mounted.
With respect to the current multistage roots pumps, it is necessary to reduce the axial length of the pump chamber and the rotors to reduce the discharge volume since the shape of rotors supported by revolving shafts are identical (for example, referring to patent document No. 1 (Japanese Patent Laid-open Publication No. 2003-307192)). However, if the axial length of rotors, i.e., the rotor thickness, is extremely thin, strength of the rotors tends to decrease thus causing deformation. Therefore, there is a lower limit for the discharge volume at the back stage.
FIG. 5 is an illustration of lobe number of rotors and discharge area. FIG. 5A is an illustration of three-lobed involute profile rotor. FIG. 5B is an illustration of four-lobed involute-toothed rotor and FIG. 5C is an illustration of six-lobed involute-toothed rotor.
The technology for solving the problem as described herein below in patent document No. 2 (Japanese Patent Laid-open Publication No. 2002-364569) is well known.
As described in patent document No. 2, the rotor at the front stage (upstream side) consists of three lobes, while the rotor at the back stage (downstream side) consists of five lobes. Through application of this kind of structure, the discharge volume is reduced by decreasing the discharge area of rotors at back stage.
Specifically, as shown in FIG. 5, regarding the widely used conventional rotor with involute-shaped teeth, wherein the radii of reference circle 01 are identical, the total discharge area (patterned area S02×4 sections in FIG. 5B) of a four-lobed rotor is approximately 78% of the total discharge area (patterned area S01×3 sections in FIG. 5A) of a three-lobed rotor, and the total discharge area (patterned area S03×6 sections in FIG. 5C) of a six-lobed rotor is approximately 53% of that of the three-lobed rotor. As a result, since it can reduce discharge area by increasing the lobe number of a rotor at back stage, it is possible to reduce discharge volume without reducing rotor thickness, as described in patent document No. 2.
Patent document No. 1: Patent Laid-open Publication No. 2003-307192 (FIGS. 8 and 9)
Patent document No. 2: Patent Laid-open Publication No. 2002-364569 (Paragraphs 0009-0015, FIG. 1-FIG. 3)
However, in traditional technology as described in patent document No. 2, there are more lobes at the back stage of the rotor, resulting in longer manufacturing time for the rotor at the back stage.
In particular, in the case of manufacturing rotors of a roots pump, a rotor cutting sheet such as a round sheet is fixed axially with good precision, and then the round sheet is cut by means of a cutting tool to make rotors in order to increase precision of distance between axial rotors. However, if the lobe number of rotors mounted on the same shaft is different, cutting operation will be complicated and it will take more time for machining.
In the case that a rotor is manufactured before fixed on the shaft, it is difficult to obtain precision of axial position. In addition, extremely high precision is required since each rotor needs to be fixed while the rotor phase is adjusted at good precision in order to secure rotor interlock on all twin rotors at multiple stages with precision. Furthermore, as described in patent document No. 2, in the case of rotors having different lobe number, interlock position is different at front stage than at back stage. Therefore, phase adjustment is particularly complicated, and it is also difficult to obtain precision and carry out assembly.